scripts/dts: extract_dts_includes: Add support for client pwms
scripts/dts: extract_dts_includes: Add support for client pwms

Add support to the extract script to handle 'pwms' client properties.
This is pretty much identical to how we hand 'gpios'.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

ldts/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,galak/zephyr,ldts/zephyr,Vudentz/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,ldts/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,Vudentz/zephyr,galak/zephyr,explora26/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,explora26/zephyr,explora26/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,punitvara/zephyr,explora26/zephyr,finikorg/zephyr,Vudentz/zephyr,punitvara/zephyr,nashif/zephyr